Technical Field
The present invention relates to a method for producing a metallic nanoparticle composite in which metallic nanoparticles are dispersed and which is usable as catalyst, for example, and also relates to a metallic nanoparticle composite produced by such a method.
Description of Related Art
An impregnation method has been used in the known art as a method for dispersing metallic nanoparticles (such as nickel nanoparticles) of a nanometer scale (e.g., having a mean particle size of 1 to 5 nm) in a porous medium (such as zeolite, mesoporous silica or activated charcoal) with microscopic pores (with a mean pore size of 2 nm or less) or mesoscopic pores (with a mean pore size of 2 to 50 nm).
According to the impregnation method, a porous medium (support) is added to an aqueous solution including a metallic element, and its water is vaporized by heating, for example, thereby loading a compound including the metallic element on the porous medium. After that, the porous medium is heated in a reducing atmosphere to reduce the loaded compound into metal.
In this case, if the metallic nanoparticles that have been loaded by the impregnation method are heated to an elevated temperature (of 300 to 400° C., for example), then those metallic nanoparticles will move and adhere and aggregate together, thus causing aggregation and sintering phenomena. As a result, those metallic nanoparticles will grow excessively. In that case, a metallic nanoparticle composite in which such excessively grown metallic nanoparticles are dispersed has such a decreased surface area exhibiting catalytic activity that the overall catalytic activity of the complex decreases, which is a problem.
Thus, to overcome such a problem, some people proposed a method of making metallic nanoparticles inside the pores of a porous medium. For example, a method of making ruthenium nanoparticles (with a mean particle size of 4 nm) inside pores of a zeolite by reducing an Ru(NH3)62+ complex introduced into the zeolite pores through ion exchange has been disclosed (see, for example, Hubert H. Nijs, Peter A. Jacobs, Jan B. Uytterhoeven, J. C. S. Chem. Comm., 1979, 1095.
Also, a method for reducing nickel ions into nickel particles (with a mean particle size of 5 nm or less) inside pores of a zeolite by making ammonium ions and nickel ions coexist as cations and by utilizing the reduction power of ammonia that desorbs itself through heat treatment has been disclosed (see, for example, Japanese Unexamined Patent Publication No. 2009-46372.
However, the method disclosed in Non-Patent Document 1 is applicable to only noble metals such as platinum and ruthenium, not to cobalt, nickel or any other base metal with high catalytic activity. The reason is that although a noble metal such as platinum or ruthenium has lower ionization tendency than hydrogen and is easily reducible, a base metal such as cobalt or nickel has higher ionization tendency than hydrogen, and therefore, is more difficult to reduce from cation sites and thus requires a lot of energy.
On the other hand, according to the method disclosed in Patent Document 1, the zeolite exhibits outstanding solid acid sites due to generation of hydrogen ions during decomposition of ammonium ions into ammonia. As a result, there will be the catalytic properties of both nickel and those acid sites there to cause a decrease in reaction selectivity and limit its use as a catalyst.